1. Field of the Invention
This invention relates to an office line interface circuit utilized in a key telephone set or the like, and more particularly an office line interface circuit designed for providing a stable characteristic for the office line impedance of the office line viewed from the side of the telephone sets irrespective of the variation in the office line condition whereby to provide an efficient sidetone suppressing characteristic.
2. Description of the Related Art
Generally in a key telephone system or the like, the office line current from the office line is not passed through the key telephone set, but instead the power is supplied to the key telephone set from an independent power source installed in a main unit. In such a key telephone system, for the purpose of suppressing a sidetone (voice signal sent from a transmitter to a receiver), a so-called sidetone preventing circuit is provided. The sidetone preventing circuit is provided with an impedance circuit whose impedance is equivalent to that of the office line viewed from the key telephone set, and the sidetone can be prevented by balancing the impedance of the impedance circuit and the impedance of the office line.
The impedance of the office line varies greatly depending upon the distance between a position at which a key telephone system, including a main unit and plurality of key telephone sets is installed and a position at which a telephone exchange is installed, whereas the impedance of the impedance circuit is generally fixed, determined in view of the manufacturing cost, etc. For example, the impedance of the impedance circuit is designed such that the sidetone preventing performance can be most efficiently manifested when the key telephone system is installed at its remotest position from the telephone exchange. Accordingly, under a specific office line condition, the sidetone preventing performance is effective, whereas under other office line conditions there is a problem that the sidetone preventing performance degrades.